The present invention relates to a method and apparatus for coating objects. In particular, the invention is concerned with a method of protecting a coating plant for coating objects with particles against danger of explosion by sparkovers obtained through electric charge of the objects to be coated.
The coating of powder or particles can provide an insulation between the object to be coated and the holding means on which the objects are connected. Consequently, the contact of the object to the respective earth potential is not guaranteed any more. Due to the present capacitance, a sparkover in form of an arc can be caused between the object and the holding means when applying high direct voltage for the coating, which sparkover has sufficient energy for igniting the particles to be coated on the object.
Methods for surveying a coating plant are already disclosed which are known under the catchword "earthing control". In these known methods, the electrical resistance between the object to be coated and the holding means is measured with low direct voltage wherein the contact resistance must not exceed a value of 1 Mohm. In case of a higher contact resistance, there is the danger that the capacity obtained by the insulated object to be coated is storing sufficient energy in connection with the high voltage applied during the coating with particles in order to ignite the powder-air mixture to be sprayed on the object when a sparkover is obtained. Especially when using small objects it proves to be rather impossible to meet the requirement with respect to the desired magnitude of the resistance. In view of the relatively small weight of the objects and the unavoidably thin insulating films which are obtained on the holding means upon the powder coating, it is not guaranteed to maintain the contact resistance below 1 Mohm so that the plant is frequently brought to a standstill.
It is further known that arc discharge having an arc energy below 5 mWs is not dangerous with regard to explosions in such powder coating plants. These arcs cannot ignite the powder of particles to be sprayed on the object, and consequently are not dangerous, so that even upon a contact resistance above 1 Mohm, which is measured with low voltage, the plant must not necessarily be stopped.